This invention relates to time-based proportional control useful for chemical dosage control of water purification systems, such as the addition of chlorine to municipal swimming pools. The invention particularly relates to improvements in a time-based proportional control system for automating dosage control.
Virtually all chemical dosage controllers marketed by U.S. Filter/Stranco within the last 10-12 years for use in the aquatic and industrial/municipal markets include a form of proportional chemical dosage control commonly referred to as Time-Based Proportional Control. Time-based proportional control has demonstrated usefulness for applications in cooling towers and other recirculated or batch systems that may experience dramatic loading swings.
Time-Based Proportional control was originally developed for use in spa applications where the chlorine demand will vary considerably due to small water volumes and the ratio of bodies/gallons of water in a spa. Standard on/off control for this application was inadequate, as the only adjustment was the chlorine feed rate. On/off only control suffers from overshooting in either low loading situations, or inability to reach setpoint in high loading situations. A common method of addressing said overshooting is to manually adjust feed rates upwardly during the day and downwardly at night.
Time-Based Proportional control is basically a variation of on/off control which utilizes a relay output. Time-based proportional control utilizes a type of pulse width modulation effective to vary the duty cycle of the output relay on-time with respect to the deviation from setpoint. The duty cycle period is typically 30 seconds to one minute. As the sensor moves farther away from setpoint, the percentage of on-time per minute will increase. Conversely, the closer the sensor moves towards setpoint, the less the feeder operates per minute. Whenever the sensor of the controller exceeds setpoint, the feeder is always off. Recent improvements to this control have been the addition of an offset to keep the feeder from completely stopping until the setpoint is exceeded by some value and the ability to change the duty cycle period.
A deficiency of the time-based proportional control function is that it fails to operate efficiently because it never reaches setpoint on high loading daysxe2x80x94when it is needed the most.
Thus, what is lacking in the art is an ability to incorporate a form of deviation compensation which could be described as time-based proportional control with automatic offset, to thus include a functionality, having a relatively small number of resets per minute and a low sensitivity which, when incorporated therein, would eliminate the systems failure to achieve setpoint on high loadings days.
The instant invention is a device and method for its use which includes the addition of a type of deviation compensation function which, in effect, provides an automatic offset ability which is effective for automatically adjusting the offset used in time-based proportional control. In practice actual offset adjustment is not always necessary. An equivalent result is achievable by including a mathematical incrementation of the duty cycle ratio. Inclusion of this improvement finds particular utility in situations like public pools where daily bather load varies depending on weather and other unpredictable factors.
Accordingly, it is an objective of the instant invention to teach a method and device to enhance time-based proportional dosing of chemical additives by inclusion of a form of deviation compensation effective to approximate an ideal proportional control response.
It is a further objective of the invention to provide an ideal proportional control response by providing time based proportional control with automatic offset.
It is yet another objective of the instant invention to provide an ideal proportional control response by mathematically incrementing the duty cycle ratio.
It is a still further objective of the instant invention to provide ideal proportional control response by including integration of a measured signal.
Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.